Many semiconductor device fabrication processes such as physical vapor deposition (PVD), high density plasma (HDP) deposition, etc., employ high vacuum chambers (e.g., base pressures before back filling with process gases of 10.sup.-8 -10.sup.-9 Torr) to affect the deposition of thin films on a semiconductor wafer. To reach such high vacuum levels after a vacuum chamber has been vented to atmosphere (e.g., for maintenance, cleaning, etc.) and to prevent film contamination due to the desorption of moisture and other gaseous elements and compounds (i.e., potential contaminants) from the chamber's interior surfaces (e.g., the chamber's shield, wafer pedestal, etc.) during elevated temperature processing, the vacuum chamber's interior surfaces must be heated to an elevated temperature (e.g., about 200.degree. C.) for a time period sufficient to desorb the potential contaminants (i.e., chamber bake-out). Improper chamber bake-out manifests itself in a degraded pre-process or "idle" chamber pressure (i.e., base pressure), an enhanced rate of pressure rise from the base pressure when the chamber's vacuum pump is shut-off (i.e., rate of rise or "ROR"), and poor deposited film quality (e.g., poor film resistivity), as described below with reference to FIG. 1.
FIG. 1 is a side diagrammatic illustration, in section, of the pertinent portions of one conventional high density plasma sputtering chamber 21. The sputtering chamber 21 contains a wire coil 23 which is operatively coupled to a first RF power supply 25. The wire coil 23 may comprise a plurality of coils, a single turn coil as shown in FIG. 1, a single turn material strip, or any other similar configuration. The wire coil 23 is positioned along the inner surface of the sputtering chamber 21, between a sputtering target 27 and a wafer pedestal 29. The wafer pedestal 29 is positioned in the lower portion of the sputtering chamber 21 and typically comprises a pedestal heater (not shown) for elevating the temperature of a semiconductor wafer supported by the wafer pedestal 29 during processing within the sputtering chamber 21. The sputtering target 27 is mounted to a water cooled adapter 31 in the upper portion of the sputtering chamber 21 so as to face the substrate receiving surface of the wafer pedestal 29. A cooling system 31a is coupled to the adapter 31 and delivers cooling fluid (e.g., water) thereto.
The sputtering chamber 21 generally includes a vacuum chamber enclosure wall 33 having at least one gas inlet 35 and having an exhaust outlet 37 operatively coupled to an exhaust pump 39 (e.g., a cryopump). A removable shield 41 that surrounds the wire coil 23, the target 27 and the wafer pedestal 29 is provided within the sputtering chamber 21. The shield 41 may be removed for cleaning during chamber maintenance, and the adapter 31 is coupled to the shield 41 (as shown). The sputtering chamber 21 also includes a plurality of bake-out lamps 49 located between the shield 41 and the chamber enclosure wall 33 for baking-out the sputtering chamber 21 as described below.
The sputtering target 27 and the wafer pedestal 29 are electrically isolated from the shield 41. The shield 41 preferably is grounded so that a negative voltage (with respect to grounded shield 41) may be applied to the sputtering target 27 via a DC power supply 43 operatively coupled between the target 27 and ground, and a negative bias may be applied to the wafer pedestal 29 via a second RF power supply 45 operatively coupled between the pedestal 29 and ground. A controller 47 is operatively coupled to the first RF power supply 25, the DC power supply 43, the second RF power supply 45, the gas inlet 35 and the exhaust outlet 37.
Although this is one chamber configuration, others, such as traditional magnetron sputter systems, which do not use a coil, or systems using hollow cathode sources, are specifically contemplated as useable with the invention herein. To bake-out the sputtering chamber 21, conventionally the bake-out lamps 49 are switched on between about 90% to 100% power when the chamber is at high vacuum. The pedestal heater (not shown) of the wafer pedestal 29 is set at about 200.degree. C., and the water supply to the adapter may or may not be shut-off. The chamber then is allowed to bake-out for about eight hours during which time degassed material will raise the chamber pressure.
For chambers in which titanium, titanium nitride or tantalum nitride are deposited, the above bake-out procedure is sufficient to produce a good base pressure (e.g., low 10.sup.-8 Torr range), a low ROR (e.g., about 10 to 20 nTorr/min), and good deposited film quality.
The reason for the success of this bake-out procedure is that both titanium and tantalum are excellent gettering materials and, therefore, once deposited on the chamber surfaces during wafer processing, can absorb (or "getter") moisture and other gaseous elements and compounds from the sputtering chamber's atmosphere. Typically, these gettered contaminants do not desorb, even during elevated temperature processing, so that the chamber's base pressure and ROR are not affected by the gettered contaminants. As well, the gettered contaminants do not significantly affect deposited film quality. An eight hour bake-out, however, results in significant process downtime for the chamber being baked-out, as well as for processing equipment upstream and downstream from the processing chamber. Overall fabrication throughput thereby is greatly degraded by conventional bake-out techniques.
When the conventional bake-out procedure is employed within a chamber for copper deposition (e.g., a copper HDP chamber) the results are less satisfactory due to copper's poor gettering properties. For instance, even after an eight hour bake-out, a high density plasma chamber for copper deposition can exhibit a high base pressure (e.g., low 10.sup.-7 Torr), a rapid ROR (e.g., about 200 nTorr/min) and a poor deposited copper film quality (e.g., poor resistivity). Accordingly, a need exists for an improved bake-out method that can be performed more rapidly than conventional bake-out methods (e.g., so as to improve throughput), and that sufficiently bakes out even a chamber for copper deposition.
A process related to and often used in conjunction with processing chamber bake-out is processing chamber cooling or "cool-down". Chamber cool-down is performed following high temperature processing or following chamber bake-out, and can result in significant process downtime for the processing chamber being cooled, as well as for processing equipment upstream and downstream from the processing chamber. For example, the time required to perform chamber maintenance and repair is initially determined by the temperature of the various chamber components which must be sufficiently cooled before handling. Opening a chamber at elevated temperatures exposes personnel to safety hazards and may result in oxidation and contamination of the chamber.
In order to mitigate the effects of contamination, chambers are typically cooled under high vacuum conditions. Because some processing chamber components are operated at temperatures in excess of 600.degree. C., cool-down time may be on the order of hours. The exact time required to reach a desired temperature depends on the chamber. For example, chamber components having high thermal conductivity (such as aluminum components) are capable of cooling more rapidly than components having low thermal conductivity (such as stainless steel components).
FIG. 2 shows a cooling curve for a typical high density plasma chamber cooled according to current practice. The chamber was operated under normal conditions and then allowed to cool under vacuum. The temperatures of a clamp ring, a coil, and a shield were measured and recorded. For comparison, the temperature of the shield was measured in two locations, zero (0) degrees from the RF feedthrough and one hundred thirty-five (135) degrees from the feedthrough. Because significant oxidation can occur at temperatures at or above 100.degree. C., the desired temperature before opening the chamber is preferably below about 50.degree. C. As can be seen from FIG. 2, the time required for all components to reach the desired temperature is at least three (3) hours. Thus, the chamber remains idle and non-productive during this cooling period plus the time required to perform the routine maintenance or repair, and to bake-out the chamber thereafter.
One attempt to cool a chamber (specifically, a Czochralski silicon growth chamber) is found in U.S. Pat. No. 5,676,751, titled, "Rapid Cooling of CZ Silicon Crystal Growth System," by Banan et al. The approach disclosed therein involves disposing a porous insulating ring within the chamber and then saturating the ring with a gas. The gas is intended to improve the thermal conductivity of the insulating ring and to provide an annular cooling medium for efficient heat exchange. Because the cooling ring is believed to transfer heat more rapidly than other chamber components the overall cooling time is reduced.
However, such an insulating system requires entirely new chambers having enlarged capacities to accommodate the insulating ring. Further, the porosity of the ring makes it unsuitable for chambers wherein process gases are needed such as CVD chambers or wherein a plasma is used such as a PVD, a CVD, or an HDP chamber. In such chambers, the process and plasma gases would be absorbed by the ring and/or outgassed during lower vacuum conditions thereby upsetting the deposition process and contaminating substrates.
Therefore, there remains a need for an apparatus and method which provide rapid cool-down of a vacuum chamber and its components from an elevated temperature so as to protect the chamber from contamination and oxidation while also ensuring the safety of personnel. Preferably, such a method may be easily adopted by existing vacuum chambers.